The present invention relates to an optical time domain reflectometry (OTDR) method and apparatus.
Optical time domain reflectometry (see Barnoski, M. K. and Jensen, S. M., Applied Optics 1976, vol. 15, pp 2112-15) involves launching a short pulse of light into an optical fibre and observing the backscatter return from the entire length of the fibre. The backscatter consists of light scattered through a variety of mechanisms, including Rayleigh, Brillouin and Raman scattering. The scattered light is quasi-isotropic and that fraction of the light which falls within the cone of acceptance of the fibre in the reverse direction is guided back towards the source. The light signal thus obtained typically takes the shape of a decaying waveform, the rate of decay being indicative of the local attenuation of the fibre. However, in addition to changes in rate of decay, localised changes in signal level can be caused by localised variations of the scattering coefficient, of the numerical aperture (for multimode fibres), or of the spot size (for single mode fibres). The distance along the fibre can be related to time of arrival of the signal by means of the known velocity of light (in a manner similar to that used in other reflectometric techniques, such as Radar or Sonar).
In the case of telecommunications applications, where OTDR is most widely used, the interest is in determining the attenuation of the fibre as a function of distance and any changes in the loss with time or position (e.g. point discontinuities).
Some of the localised effects could be caused by the action of external measurands and this fact has been exploited in a variety of designs of distributed sensor (see Hartog, A. H., J. Lightwave Technology, 1983, vol. LT-1, pp 498-509). In those designs which have been developed commercially, a small part of the scattered light spectrum, consisting of Raman or Brillouin scattered light, is selected. These spectral lines are typically very weak compared with the dominant Rayleigh scattering, and a major problem in the design of such sensors is achieving a sufficient signal-to-noise ratio to obtain a measurement of adequate resolution in an acceptable measurement time.
In the cases of both OTDR and of distributed sensing using OTDR, one major limitation is that of the power which can be launched into the fibre. The performance of optical time domain reflectometers (OTDRs) and OTDR-based sensors is measured by the maximum length of fibre which can be measured to a given signal uncertainty in a given measurement time with a given spatial resolution. The length of fibre is itself determined by the ratio of the dynamic range of the instrument to the loss per unit length of the fibre measured. Since the losses vary between fibres, a more common description of the range of an OTDR is the dynamic range, i.e. the maximum one-way signal attenuation at which the backscatter signal(s) can be measured to the required resolution.
The dynamic range is determined principally by the energy of the probe pulse launched into the fibre, the sensitivity of the receiver and, although these cannot always be controlled by the instrument designer, by the characteristics of the fibre and the efficiency of the optical arrangement within the instrument. Thus the range of an OTDR or OTDR-based sensor is maximised by making the receiver as sensitive as possible and launching as much energy as possible into the fibre. The energy of the pulse may be increased by increasing either its peak power or its duration. In the latter case, the spatial resolution of the instrument (i.e. its ability to distinguish separate, but closely adjacent, features along the fibre) is degraded. The central problem is thus one of increasing the energy launched into the fibre without degrading the spatial resolution.
Whereas the technology of semiconductor lasers until recently limited the power available within optical fibres, especially single-mode fibres, to approximately 100 mW, the development in recent years of optical amplifiers, especially those based on rare-earth-doped fibres, has lifted this limitation for all practical purposes, at least in pulsed applications. The power which may be launched into an optical fibre is therefore limited by non-linear effects, which result from the interaction of high-intensity light with the glass forming the structure of the fibre. Optical non-linear effects occur at modest power levels in optical fibres because the guiding structure confines the optical power to a small area over very long distances, resulting in far greater interaction lengths than could be achieved with Gaussian beams in a non-guiding medium. These non-linear effects have been reported in a number of publications and are summarised below, but a more detailed review may be found (for example) in Chapter 10 of the book by K. T. V. Grattan and B. T. Meggitt (Eds.): xe2x80x9cOptical Fiber Sensor Technologyxe2x80x9d Chapman and Hall 1995 (ISBN-0-412-59210-X).
a) Stimulated Raman Scattering (SRS):
The stimulated Raman effect results from the interaction of the incident radiation with molecular vibrations (optical phonons) and gives rise to the conversion of optical power from the incident wavelength to (in the first instance) a longer wavelength, known as the Stokes wavelength. The Stokes wavelength is separated from the incident wavelength by a frequency shift, which depends on the materials forming the fibre, but for silica-based fibres is mainly around 440 cmxe2x88x921. Thus for incident light at 1550 nm, the first Stokes radiation appears at a wavelength of about 1663 nm.
For a probe wavelength of 1550 nm launched into a long length (of order 5 km or more) of single mode fibre of the type commonly used for telecommunications purposes, the stimulated Raman effects converts significant amounts of probe power to the first Stokes wavelength when the peak power exceeds typically 1 to 3 W, depending on the design of the fibre. If the optical power at the Stokes wavelength builds up to a sufficient level, it can itself generate light at a second Stokes wavelength and so on. Under suitable conditions, SRS can also occur at a shorter wavelength (anti-Stokes Stimulated Raman scattering), but the predominant effect is a shift to longer wavelength, which can be so efficient that most of the power of the incident light is converted to longer wavelengths.
Stimulated Raman scattering is primarily a forward-effect (i.e. the converted light travels in the same direction as the incident light) and is determined by the peak optical power. It is relatively independent of the duration of the pulse. It is also independent of the spectral width of the incident light, provided the latter falls within the broad gain spectrum of the Raman process (in the case of an incident wavelength of 1550 nm, the incident spectrum would scarcely affect the efficiency of the SRS process until it reached some 35 nm full-width at half maximum).
b) Stimulated Brillouin Scattering (SBS):
Stimulated Brillouin Scattering is caused by the interaction of the incident light with lattice vibrations (acoustic phonons), particularly those which have an acoustic wavelength similar to the incident optical wavelength. Like SRS, it results in the generation of a new wavelength, shifted with respect to the incident wavelength by a frequency equal to that of the acoustic phonons taking part in the interaction. This frequency depends on the material and the incident wavelength, but in silica-based fibres and for an incident wavelength of 1550 nm it is typically 10.7 GHz.
Unlike SRS, SBS is a very narrow-linewidth process. Thus if the incident illumination has a broader spectrum than that of the process (typically 100 MHz in silica-based fibres), then the threshold for efficient conversion is raised in proportion to the ratio of the linewidth of the source to that of the SBS process. A further difference between SRS and SBS is that the latter is primarily a backward process, i.e. the new wavelength travels in the reverse direction from that of the incident radiation. As a result, the overlap between the incident light and the Brillouin emission occurs only over a length of the fibre corresponding to the pulse width. The threshold for stimulated Brillouin scattering is therefore proportional to the product of pulse power and pulse duration, i.e. to the energy in the pulse. For continuous-wave input power launched into low-loss fibres, the SBS effect can occur at extremely low optical power, of order 1 mW.
c) Other Mechanisms:
There exist other effects which, under certain circumstances, can limit the allowable power launched in the fibre, such as four-wave mixing and self-phase modulation. However, they are of relevance primarily to OTDRs where coherent detection methods are employed.
At a wavelength in the region of the lowest loss transmission for silica-based fibres, 1550 nm, the limitation caused by Raman scattering on allowable transmitted power is around 1 W, depending on the loss of the fibre and its design. As noted earlier, the power limitation for stimulated Brillouin scattering depends on the pulse duration and for 1 m spatial resolution (i.e. 10 ns pulse duration) it is similar to that for Raman scattering, for an incident linewidth significantly below that of the scattering process. For longer pulses, the maximum allowable power decreases in proportion to the pulse duration.
Since it is desirable to increase the dynamic range of both OTDRs and OTDR based sensors, a number of methods have been employed and described in the literature. Some of the methods employed are described below, but a more detailed review may be found (for example) in the book by K. T. V. Grattan and B. T. Meggitt (Eds.): xe2x80x9cOptical Fiber Sensor Technologyxe2x80x9d Chapman and Hall 1995 ISBN 0 412 59210 X (where Chapter 11 is primarily of relevance to the present application and is hereby incorporated by reference).
In order to increase the energy launched into the fibre without degrading the spatial resolution, methods based on pulse compression coding [Healey, P., Proc. 7th European Conf. on Optical Communication, Copenhagen 1981, pp 5.2.1-4; Bernard, J. J. et al, Symposium on Optical Fiber Measurements, Boulder, Colo. NBS Publication 683 m pp95-8; Bernard, J. J. and Depresles, E. Proc. S.P.I.E., 1987, vol. 838, pp206-9; Everard, J. K. A., Electronics Letters 1989, vol. 25, pp 140-2] and variants thereof have been employed. 
In essence, a train of pulses, either continuous or of finite code length, is launched into the fibre. This gives rises to a number of separate backscatter waveforms which overlap in time when returning to the OTDR instrument. These overlapping waveforms are detected and the electrical output thus obtained is fed to a correlator circuit, together with the original code. The output of the correlator is a signal having a spatial resolution similar to that of a single pulse in the input pulse train, but an intensity increased by the number of xe2x80x9c1xe2x80x9d pulses in the pulse train. In this way, the signal-to-noise ratio is increased by a factor given by half the square root of the number of xe2x80x9c1xe2x80x9d pulses in the input code. This improves the resolution of the backscatter measurement without, in principle, degrading the spatial resolution.
The limitations of these methods are firstly that there normally remain xe2x80x9csidelobesxe2x80x9d in the correlation function, i.e. that, even under ideal conditions, features occurring in one part of the fibre re-appear in other parts, albeit with much lower intensity. Secondly, where the code is continuous (as is the case when an m-sequence is used), the signal-dependent noise arising from the strong near-end backscatter dominates the very weak signals generated at the remote end of the fibre. Since, for a continuous code, the near-end noise arrives at the receiver simultaneously with the far-end signal, this cannot be eliminated by its time of arrival as is the case in single-pulse reflectometry.
Unfortunately, the use of finite-duration codes does not solve the problem just discussed, since such finite-length codes have worse side-lobe characteristics than continuous codes.
Complementary code methods, in which several different codes are launched into the fibre, such that the sum of their correlation functions is the ideal single-pulse response function, have been proposed and commercial equipment based on these principles have been manufactured [Nazarathy, M. et al. J. Lightwave Technology, 1989, vol. LT-7, pp24-38]. However, a further, practical, problem remains with these methods, namely that, for the correlation processing to be effective, it is necessary for the-optical pulse train to be extremely accurate, i.e. that the energy in each of the laser pulses must be uniform and their position exactly as required by the definition of the pulse sequence. In practice, these conditions are difficult to meet and failure to generate the required optical waveform accurately results in additional sidelobes in the correlation function. A further requirement in such systems is that all of the detection and signal acquisition electronics should exhibit a very high degree of linearity.
Related methods, such as frequency-modulated continuous wave modulation [Venkatesh, S. and Dolfi, D. W. Applied Optics, 1990, vol. 29, pp1323-6], or step-frequency modulation [MacDonald, R. I., Applied Optics 1981, vol. 20, pp1840-4] derived from Radar technology have also been proposed. So far few of the latter methods for increasing the pulse energy have been successful commercially, owing to the difficulties mentioned.
Alternative approaches for extending the range of OTDRs and OTDR-based sensors have involved optimising receiver and optical design, and a detailed review of these may be found in the book by K. T. V. Grattan and B. T. Meggitt (Eds.): xe2x80x9cOptical Fiber Sensor Technologyxe2x80x9d Chapman and Hall 1995 ISBN 0 412 59210 X (see particularly Chapter 11).
One such approach involves the use of coherent optical receivers [Healey, P. et al., Electronics Letters 1982, vol. 18, pp862-3], where the detector is illuminated, in addition to the backscattered light, with the output of a local oscillator which emits at the same frequency as the backscattered light (in the case of homodyne detection) or is frequency-shifted relative to the backscatter (in the case of heterodyne detection). In either case, the detector responds to the product of the electric fields of the local oscillator and of the backscattered light. Since the local oscillator power can be made arbitrarily large, it is possible to ensure that this product dominates the noise of the receiver. In principle, detection sensitivities approaching the quantum limit can be obtained. Coherent detection OTDR requires sources of very narrow linewidth, which has resulted in the method receiving relatively little practical application in the 15 years since it was initially proposed. However, the sources and other components required for assembling a coherent OTDR have advanced in the intervening years and further results have been published recently.
Several practical difficulties, however, affect this method. The first is that the interference effect, which takes place at the detector and which provides the frequency down-conversion inherent in coherent detection, is polarisation-sensitive. Although the state of polarisation of the local oscillator may be controlled, that of the backscatter is affected by the birefringence of the sensing (or measurement) fibre, which in turn is dependent on the external environment of the fibre. Therefore it is necessary to use a polarisation-diversity receiver (i.e. a dual receiver which is simultaneously and separately sensitive to two orthogonal states of polarisation of the backscatter); alternatively, the polarisation of the local oscillator must be varied continuously over an adequately wide sample of all possible polarisation states during each measurement to xe2x80x9cscramblexe2x80x9d the relative polarisations of signal and local oscillator. A second problem [Healey, P. Electronics Letters 1984, vol. 20, pp30-2 and Healey et al, ibid., 1984, vol. 20, pp360-2] is that of xe2x80x9ccoherencexe2x80x9d fading, where the backscatter signal, when generated by a very narrow-band source, is found to interfere with itself. The result is a stable envelope for the backscatter signal, under which the signal can vary, at usually a very slow rate, from 0% to 100% of the envelope. Clearly, this fading introduces significant errors in the acquisition of the backscatter signal and the only solution found to date has been to vary the input optical frequency over as wide a range as possible during the measurement.
A further enhancement to the coherent detection method has recently been proposed and demonstrated by M. J. Sumida, in Lightwave Technology, 1996, vol. 11, pp 2483-91. An OTDR employing this method is known as a M-ary coherent OTDR. In this arrangement of a heterodyne OTDR, a sequence of M pulses, each shifted in frequency with respect to one another and with respect to the laser source from which they are derived, is launched into the fibre under test. The backscatter return from this sequence of pulses is combined with the unshifted, continuous, laser output (which serves as a local oscillator) onto a single optical receiver.
The receiver output consists of M signals at each of the difference frequencies between on the one hand the local oscillator and on the other hand each of the probe pulses. In the apparatus, these frequencies are separated by means of a series of M bandpass filters. Each of the M frequencies is then down-converted by a frequency-translation circuit and then processed through a separate envelope detection circuit. The resulting rectified signals are then time-shifted relative to one another, to cancel the relative delays between the times at which each of the M pulses were launched into the fibre; each of these signals represents a separate measurement of the backscatter in the fibre. The time-aligned signals are then added together, which is equivalent to summing the outcome of separate measurements using individual pulses (in practice, it is easier first to digitise the signals emerging from the envelope detection circuits using separate analog-to-digital conversion circuits and to carry out the subsequent manipulation in digital circuitry).
This method has several claimed benefits, namely that it improves the measurement time required to achieve a pre-determined signal-to-noise by a factor of M. Secondly, because the fading effects are extremely dependent on the exact frequency of the probe pulse, the quasi-simultaneous measurement by M pulses has the effect of substantially reducing the fading.
However, the M-ary coherent detection approach is appropriate only to Rayleigh backscattering systems, which would exclude many distributed sensor designs. A further drawback of the M-ary coherent detection method is that it requires an M-fold replication of a considerable number of electronic circuits. Moreover, for a reasonably high-resolution OTDR or OTDR-based sensor and a large number of pulses concurrently launched into the fibre, the frequency range to be covered by the receiver, bandpass filters and frequency translation circuits is substantial. In the example given by the author for an extension to 100 pulses, the frequency range would have been up to 8 GHz. Thus whilst the M-ary coherent detection method may find applications in long-distance telecommunications where the spatial resolution required is modest, it is expensive and complicated for sensor applications and not so well suited to high resolution OTDR applications.
Accordingly, it is desirable to provide a more practicable method for overcoming the peak power limitations of optical fibres, applicable to a wide range of OTDRs and OTDR-based sensors.
In particular, it is desirable to increase the probe energy launched into the fibre, whilst avoiding detrimental effects, such as optical non-linearity or loss of spatial resolution. According
According to a first aspect of the present invention there is provided an optical time domain reflectometry method in which a plurality of pulses of optical radiation, having different respective wavelengths and delayed by known amounts of time relative to one another such that adjacent pulses do not overlap, are launched into an optical fibre of interest and optical radiation backscattered from the fibre is detected to produce electrical output signals, the said backscattered optical radiation being processed before detection so as to remove the effect thereon of the relative time delays between the said plurality of pulses; characterised in that the said plurality of pulses is derived from a pulse of optical radiation in a preselected wavelength band, each of the said pulses having a peak power less than the power at which non-linear effects begin to occur in the said optical fibre.
The said pulse of optical radiation in a preselected wavelength band may be emitted by a broadband source containing a plurality of different wavelengths.
There are n pulses in the said plurality, where n is an integer greater than or equal to 2.
A method embodying the first aspect of the present invention may be used in characterising the said optical fibre of interest.
Alternatively, a method embodying the first aspect of the present invention may be used for sensing respective values of a physical parameter at different locations along the said optical fibre of interest.
In one example of the method applied to sensing, the part of the said backscattered optical radiation which is used to produce the said output signals comprises that in respective spectral bands resulting from spontaneous Brillouin scattering in the optical fibre of the said plurality of pulses of optical radiation.
According to a second aspect of the present invention there is provided optical time domain reflectometry apparatus comprising means operable to launch a plurality of pulses of optical radiation, delayed by known amounts of time relative to one another such that adjacent pulses do not overlap, into an optical fibre of interest and detection means operable to produce electrical output signals in response to optical radiation backscattered from the fibre; characterised by: source means for emitting a pulse of optical radiation in a preselected wavelength band; pulse spreading means, connected to receive the said optical radiation emitted from the said source means and operable to derive therefrom the said plurality of pulses such that the pulses of the said plurality have different respective wavelengths and a peak power less than the power at which non-linear effects begin to occur in the said optical fibre, the said pulse spreading means being connected to launch the said plurality of pulses into the said optical fibre; and pulse re-forming means connected, between the said optical fibre and the said detection means, to intercept backscattered optical radiation from the said optical fibre, the said pulse re-forming means being operable to process said backscattered optical radiation so as to remove the effect on that backscattered optical radiation of the relative time delays between the said plurality of pulses and to output the processed backscattered optical radiation to the said detection means.
The said source means may be a broadband source containing a plurality of different wavelengths.
There are n pulses in the said plurality, where n is an integer greater than or equal to 2.
In some cases, the said pulse spreading means desirably also constitute the said pulse re-forming means.
The said pulse-spreading means and/or the said pulse re-forming means desirably comprise a serial reflective network.
The said serial reflective network may comprise a plurality (n) of serially-connected wavelength-selective reflectors.
The said reflectors advantageously comprise gratings.
At least some of the reflectors are preferably connected by amplifying fibre sections.
Desirably, when the pulse-spreading means are distinct from the said pulse re-forming means, the said reflectors of the pulse-spreading means have different spectral widths to the reflectors of the pulse re-forming means.
In this case, the spectral widths of the reflectors of the said pulse spreading means are narrower than those of the reflectors of the pulse re-forming means.
At least some of the said reflectors of the pulse-spreading means may be selected so as to have differing respective spectral widths such that all of the pulses derived by the pulse-spreading means are of approximately equal amplitude.
The said pulse-spreading means and/or the said pulse re-forming means may alternatively comprise a parallel transmissive or reflective network.
The said parallel transmissive network preferably comprises a wavelength division demultiplexer, a wavelength division multiplexer and a plurality (n) of fibre delay lines connected between the said wavelength division demultiplexer and the said wavelength division multiplexer.
The said parallel reflective network may comprise a wavelength division demultiplexer and a plurality (n) of fibre delay lines connected between the said wavelength division demultiplexer and reflective means.
Apparatus embodying the second aspect of the present invention is designed so as to substantially avoid the occurrence of non-linear effects in the pulse-spreading means.
This is preferably achieved by restricting the peak power of optical radiation entering the said pulse spreading means to a level below that at which non-linear effects occur, in which case the apparatus further comprises amplifying means for amplifying the pulses produced by the said pulse spreading means.
Apparatus embodying the second aspect of the present invention is advantageously used for sensing respective values of a physical parameter at different locations along the optical fibre, in which case the said optical fibre is deployed through a region of interest.
Preferably, the said detection means are operable to detect backscattered optical radiation in respective spectral bands resulting from spontaneous Brillouin scattering in the optical fibre of the said plurality of pulses of optical radiation.
The said source means desirably emit optical radiation in more than one spectral band, such that each spectral band is narrow and is separated from the or each of the others such that the spectral bands resulting from spontaneous Brillouin scattering in the fibre remain distinct from the Rayleigh spectral band and other spectral bands present in the source means.
The said source means are preferably formed by a laser cavity defined between a pair of mirrors and further comprising, in series between the said mirrors, a gain medium, a Q-switch device and a wavelength-selective element having at least two pass bands.
The source means desirably emit a spectrum in which the spectral bands are substantially periodic. The spectral bands may advantageously have a period which is approximately twice the Brillouin wavelength shift.
Preferably, the wavelength-selective element also serves to separate out the Brillouin backscatter signals and the Rayleigh backscatter signals.
Thus, in an embodiment of the present invention, enhanced performance may be obtained by selecting a pulsed source the output of which covers a relatively wide spectral range. The source spectrum is divided by an optical network into at least two spectral bands, which are delayed relative to one another and launched into the fibre. The resulting backscattered waveforms are processed by means of an optical network which cancels the relative delays between the constituent spectral bands. The optical signal thus obtained is then fed to an optical receiver which converts the signal into electrical form which is then processed in the usual way.